Would be nice if the global $blog_id variable for a single site installation would default to the domain name listed in Site Address or WordPress Address from the database, rather than having a default value of 1. Using the domain name as the $blog_id variable would allow multiple single site installations on the same server to have a unique default $blog_id, and to use Memcached with this Memcached plugin without the need to set a global $blog_id variable in wp-config.php. A similar approach to having a unique globabl $group variable would also be helpful for the same reason if the batcache plugin is used.

Even after the changes above, single site installations still have data that gets replaced when it should remain persistent. Only Memcached plugin version 2.0 works properly with multiple single site installations on the same server, unless there is something I am missing.

Yeah I missed something. My first ticket, and I miss something. The problem is the table prefix is not unique, so Don Daniello solved the problem by using part of the NONCE_SALT constant as a unique superprefix. This might be an issue only for older installations that having defined keys and salts. I would prefer to have a unique password key generated and stored in the database to eliminate the need for wp-config.php to be edited, in case the web host doesn't allow writing to a file.

Trac is not the correct venue for this, but the lead developer of WordPress is also the developer of the Memcached plugin, and we also use it on WordPress.org etc. I've sent the link to ryan to ensure he sees it.

I agree with the change in the general sense. $blog_id should probably be respected for caching reasons. However, if you have multiple individual installations, you're already going to run into a potential issue with global groups, especially if they have the same table prefix (different databases). Ultimately, just be smarter about your table prefixes and ensure they are unique across a server in which you are using a shared memcached pool, and you will be all set.